As shown in U.S. Pat. Nos. 3,680,002, 3,702,971 and 3,882,420, microwave oscillators may be tuned in several ways. For example, they may be tuned by altering electrical capacitance or by altering induced magnetic field flux densities.
Some microwave osciallators employ a small slab of dielectric material, referred to as a dielectric resonator, adjacent an electrical conductor in order to create resonance at a selected frequency of current carried by the conductor. The resonant frequency of the dielectric resonator is dependent upon the geometry of the conductors within its vicinity. This fact has been used in devising means for manually adjusting the resonant frequency as by turning a conductive screw or the like so as to alter its proximity or orientation with respect to the dielectric resonator.
Today, microwave oscillators have become miniaturized and designed to be used as surface mounted devices (SMD). This miniaturization has made it highly desirable to have the oscillators hermetically sealed so that moisture and airborne particles may not effect their operation. This reduction in scale and attendant requirement for sealing has made it extremely difficult to devise practical means for tuning the oscillators. For example, mechanically tuning the devices by moving a conductor in the vicinity of a dielectric resonator, or another conductor within the oscillator, is highly impractical and unreliable in itself since extremely small changes in position are required and since vibrations tend to reposition the movable conductor.
Hermetically sealing presents further design difficulties. For example, one could consider placing the oscillator within an expandable bellows that bears conductive members but such would tend to "microphone" when vibrated. One could tune oscillators electronically as with the use of varactors. However, this produces low Q resonance since varactors tend to load oscillator circuits. Frequency multipliers could also be used but they are complex, expensive and generally unreliable.
Accordingly, it is seen that a strong need exists today for sealed, tunable, microwave oscillators that could be practically used as surface mounted devices and which could be externally tuned in a simple, predictable and reliable manner. It is to the provision of such an oscillator therefore that the present invention is primarily directed.